AP Physics 1 key terms

A vector giving the straight-line change in position from a starting point to an ending point, including direction. It differs from distance traveled, which counts total path length without regard to direction.

The displacement of an object divided by the time interval over which that displacement occurred. It is a vector quantity and ignores any changes in speed or direction during the interval.

The velocity of an object at one specific moment, equal to the slope of the position-versus-time graph at that instant. It is the limit of average velocity as the time interval shrinks toward zero.

The rate at which velocity changes with time, including changes in speed, direction, or both. It is a vector, and its sign relative to velocity determines whether an object speeds up or slows down.

Motion under the influence of gravity alone, with air resistance ignored. Near Earth's surface the acceleration is about 9.8 meters per second squared downward, regardless of the object's mass.

Two-dimensional motion of an object launched into the air and then moving only under gravity. Horizontal velocity stays constant while vertical velocity changes at the acceleration due to gravity, producing a parabolic path.

The tendency of an object to resist changes in its state of motion. Mass is the quantitative measure of inertia, so a more massive object requires a larger net force for the same acceleration.

The net force on an object equals its mass times its acceleration, written as the vector equation F_net = ma. Acceleration points in the same direction as the net force.

The single vector sum of all individual forces acting on an object. It determines the object's acceleration; a zero net force means the object's velocity does not change.

A sketch showing one object in isolation with arrows representing every external force acting on it. It is used to find the net force and set up Newton's second law equations.

The perpendicular contact force a surface exerts on an object resting against or pressing into it. Its magnitude adjusts to the situation and is not always equal to the object's weight.

A contact force that opposes relative sliding, or the tendency to slide, between two surfaces. Kinetic friction acts during sliding, while static friction can vary up to a maximum value.

A dimensionless number describing how strongly two surfaces resist sliding against each other. Multiplying it by the normal force gives the maximum static friction or the kinetic friction magnitude.

The pulling force transmitted along a rope, string, or cable when it is stretched. In an ideal massless string over a frictionless pulley, tension has the same magnitude throughout.

The net force directed toward the center of a circular path that keeps an object moving in that circle. It is not a new kind of force but a role played by tension, gravity, friction, or another force.

The acceleration of an object moving in a circle, directed toward the center and having magnitude v squared divided by the radius. It changes the direction of velocity, not its magnitude.

Motion along a circular path at constant speed. Because the direction of velocity keeps changing, the object is accelerating toward the center even though its speed is constant.

The energy transferred to or from an object by a force acting through a displacement, equal to the product of force, displacement, and the cosine of the angle between them. Only the force component along the displacement does work.

The energy an object possesses because of its motion, equal to one-half its mass times its speed squared. It is a scalar and is always zero or positive.

The net work done on an object equals the change in its kinetic energy. This lets you relate forces and displacements directly to changes in speed without tracking time.

Energy stored in a system because of the arrangement or relative positions of its parts, such as an object raised above the ground or a stretched spring. It converts to kinetic energy as the configuration changes.

The principle that total energy in an isolated system stays constant, though it may change form. For mechanical systems without friction, the sum of kinetic and potential energy remains constant.

A force such as gravity or an ideal spring force for which the work done depends only on the starting and ending positions, not the path taken. It allows a potential energy to be defined.

The rate at which work is done or energy is transferred, measured in watts. It equals work divided by time, or the product of force and velocity when the force and motion are aligned.

A vector equal to an object's mass times its velocity, describing how hard it is to stop. Changes in momentum are caused by impulse, the product of net force and time.

The product of a net force and the time interval over which it acts, equal to the change in an object's momentum. Extending the collision time reduces the peak force for a given momentum change.

The principle that the total momentum of an isolated system stays constant when no external net force acts. It is used to analyze collisions and explosions, often combined with energy considerations.

A collision in which both momentum and kinetic energy are conserved. Objects bounce apart with no loss of total kinetic energy to other forms.

A collision in which momentum is conserved but total kinetic energy is not, because some energy converts to heat, sound, or deformation. In a perfectly inelastic collision the objects stick together.

The point representing the average position of all mass in a system, which moves as if all external forces acted there. Internal forces do not change the motion of the center of mass.

A measure of how effectively a force causes rotation about an axis, equal to force times lever arm times the sine of the angle between them. It increases with greater force or greater distance from the pivot.

The rotational equivalent of mass, describing how mass is distributed about a rotation axis and how strongly an object resists angular acceleration. It depends on both the object's mass and its shape.

The rate at which angular velocity changes with time, measured in radians per second squared. It is produced by a net torque and is inversely related to the moment of inertia.

The condition in which the net torque on an object is zero, so its angular velocity does not change. Combined with zero net force, it is used to solve static problems involving beams and supports.

A measure of rotational motion equal to moment of inertia times angular velocity, or for a point mass, mass times velocity times perpendicular distance from the axis. It is conserved when the net external torque is zero.

The principle that the total angular momentum of a system stays constant when no net external torque acts on it. A spinning skater pulling in her arms speeds up because her moment of inertia decreases.

The energy an object has because it is rotating, equal to one-half its moment of inertia times the square of its angular velocity. A rolling object has both rotational and translational kinetic energy.

Oscillation in which the restoring force is proportional to displacement and directed toward equilibrium, producing a sinusoidal motion with constant period. A mass on an ideal spring and a small-angle pendulum approximate it.

The time required to complete one full cycle of a repeating motion, measured in seconds. For simple harmonic motion it is independent of amplitude and depends on the system's mass and stiffness or length.

The number of complete oscillations per second, measured in hertz and equal to the reciprocal of the period. A higher frequency means a shorter period.

The maximum displacement of an oscillating object from its equilibrium position. It sets the energy of the oscillation but, for an ideal simple harmonic oscillator, does not affect the period.

A value describing the stiffness of a spring, equal to the force needed per unit of extension or compression. It appears in Hooke's law and in the formulas for a spring oscillator's period and potential energy.

The mass of a substance per unit volume, usually expressed in kilograms per cubic meter. It determines whether an object floats or sinks in a fluid.

The force exerted perpendicular to a surface divided by the area of that surface, measured in pascals. In a fluid at rest, pressure increases with depth.

The upward force a fluid exerts on an object immersed in it, equal to the weight of the fluid displaced. An object floats when this force can match its weight.

The statement that the buoyant force on an object equals the weight of the fluid it displaces. It explains floating and apparent weight loss when submerged.

For an incompressible fluid flowing through a pipe, the product of cross-sectional area and flow speed is constant. Fluid speeds up where the pipe narrows and slows where it widens.

A relationship stating that along a streamline in steady, ideal flow, the sum of pressure, kinetic energy per unit volume, and gravitational potential energy per unit volume stays constant. Faster flow corresponds to lower pressure.